The present invention relates to novel filled silicone compositions useful in the preparation of acoustic lenses for ultrasonic imaging or therapy devices.
The acoustic impedance of a typical silicone resin is relatively low, about 1 MRayls compared to the acoustic impedance of human tissue, which is typically about 1.5 MRayls. For purposes of acoustic lenses used in ultrasound probes for medical imaging, the mismatch in acoustic impedance between the silicone resin comprising the acoustic lens component of the ultrasound device and the patient's tissue results in reduction of ultrasound transmission efficiency between ultrasound probe and human body. The acoustic impedance of the silicone resin used as lens material can be increased and brought closer to acoustic impedance of human tissue by incorporation of higher density inorganic fillers. Many inorganic fillers such as fumed silica, quartz, TiO2 and Al2O3 have been employed in an effort to increase the acoustic impedance of silicone resins useful in the preparation of acoustic lenses for ultrasound probes. Due to relatively low density (from about 2.2 to about 4.2 g/cm3) of these metal oxides, a high filler loading (from 20 to 40 vol %) is required to attain acoustic impedances of 1.5 MRayls or higher. High filler loading results in undesirable properties of the filled silicone composition, for example high viscosity, difficult processing properties, and high acoustic attenuation. Recently, Yamashita and his coworkers reported several formulations of filled silicone compositions comprising high-density fillers such as heavy metals, and submicron-sized particles of heavy metal oxides. Filled silicone compositions disclosed by Yamashita exhibited acoustic impedance values of about 1.5 MRayls and were reported to comprise nano-particulate platinum metal and ytterbium oxide. Recent progress in the field of materials useful for the preparation of acoustic lenses for ultrasound probes notwithstanding, further improvements were desirable and are disclosed herein.